1. Field of the Invention
The present invention relates to a DC-DC converter circuit for use in generating a power source voltage.
2. Description of the Related Art
Portable electronic devices use batteries (dry batteries, secondary batteries, and so on) as their power sources. In personal headphone stereos, for example, two batteries are usually used to power 3V types and one battery to power a 1.5 V type, the latter becoming more popular.
Such low voltage power sources, however, may often be insufficient to fully operate the respective circuits of the devices. Therefore, a booster circuit or the like, which obtains a high voltage by quickly switching an electric current in a coil, using a switching transistor, may be used, and the obtained high voltage is used as a power source voltage of a circuit.
In particular, a headphone stereo incorporating a radio receiver mixes a local frequency signal from a local oscillator and a received RF signal to obtain an IF (intermediate frequency). This requires a local frequency signal according to a frequency of a desired station signal, and, for this purpose, a varactor diode, capable of changing its capacity depending on a voltage, may often be employed. For changing a varactor diode capacity, a voltage of about 12 V is necessary, which may be generated using a booster circuit.
Here, a booster circuit uses a switching transistor for switching on or off a coil current. Generally, various circuits are provided in the form of a semiconductor IC having a transistor, of which compressive strength is about 15 V. However, as an output voltage of the booster circuit increases, a voltage greater than 15 V may be applied to the transistor.
Then, a zener diode with a breakdown voltage of 12 V may additionally be provided parallel to a transistor. A zener diode, however, may quickly flows a current to the ground when it is supplied with a voltage greater than the breakdown voltage. This current may cause a noise in the boosted voltage, resultantly varying a local frequency.
Further, the transistor is switched at a relatively high speed. Should a difference between the switching frequency or harmonic of the switching frequency and the frequency of a received signal fall in an audible frequency band, beat interference may be caused.
The present invention aims to provide a DC-DC converter circuit which prevents variation in a boosted voltage and suppresses occurrence of beat interference.
According to the present invention, there is provided a DC-DC converter circuit, comprising a coil one end of which is connected to a battery; a transistor one end of which is connected to the other end of the coil, and the other end of which is connected to a ground, for being switched on or off in response to a predetermined clock signal; a zener diode connected at an anode thereof to the other end of the coil, for outputting via a cathode thereof a boosted voltage; and a capacitor connected to the cathode of the zener diode, for maintaining the boosted voltage, wherein the breakdown voltage of the zener diode is set according to compressive strength of the transistor.
With this arrangement, a voltage to be applied to the transistor can be limited by setting a breakdown voltage of the zener diode, so that the transistor can be prevented from being destroyed. In addition, as the zener diode prevents the breakdown current from flowing directly to the ground, variation of the boosted voltage can be prevented.
Further, preferably, the boosted voltage is used as a power source voltage for adjusting a local frequency of a radio receiver, and the predetermined clock signal for switching on or off the transistor in receiving AM broadcasting is a clock obtained by dividing the local frequency. With this arrangement, a difference between a transistor switching frequency and the frequency of a desired station can be always kept out of the audible frequency band, so that occurrence of beat interference can be prevented.
Further, preferably, the predetermined clock signal for switching on or off the transistor in receiving FM broadcasting is a circuit operation main clock or a clock obtained by dividing a circuit operation main clock. For FM broadcasting, the transistor switching frequency is significantly different from the frequencies of FM broadcasting stations. Therefore, with use of a main clock intact or a clock obtained by dividing a main clock, as a transistor switching clock signal, an FM radio receiving circuit having a simpler structure can be realized.
Still further, preferably, the boosted voltage is used as a power source voltage for adjusting a local frequency of a radio receiver, and the predetermined clock signal in receiving FM broadcasting is a clock obtained by dividing the local frequency. When a sufficient voltage cannot be obtained at an output of the booster circuit while receiving FM broadcasting, using a signal obtained by dividing the local frequency allows sufficient boosting.